The following discussion of the background of the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge in any jurisdiction as at the priority date of the application.
Light Emitting Diodes (LEDs) are often driven by controller circuits to fulfill one or more objectives. As LEDs are sensitive to current and voltage fluctuations, one of the most important objectives for LED controller circuits is to provide appropriate current to drive the LEDs under supply voltage or load variations.
Pulse-Width Modulation (PWM) based controllers have been developed to provide appropriate current to LEDs under supply voltage/load variations. Most PWM controllers are switched-mode based controllers involving the use of electronic switches such as MOSFET as a means for regulating the current input to the LEDs.
FIG. 1 illustrates a typical buck-boost LED controller used for an isolated AC application. The current driver circuitry is based on an analog PWM controller, which includes the use of current feedback resistors 13; opto-couplers 14; and polarized capacitors 12. The current driver circuitry further includes an analog PWM controller 15.
The PWM based controller circuit has several disadvantages listed as follows:
As the current-sense feedback resistors 13 are in the current path of the power MOSFET and the LEDs, it dissipates a lot of electrical energy. This is a loss of electrical energy as the feedback resistors 13 consume electrical energy (which is dissipated as heat) which is not producing useful work. Moreover, such feedback resistors 13 are typically power resistors which generate considerable heat and generally require thicker copper design when implemented on a printed circuit board (PCB). This increases the overall cost of implementation.
The lifetime of the polarized capacitors 12 is typically much shorter than the lifetime of the LEDs. When one or more polarized capacitors 12 break down, they lead to short circuits which may further cause the malfunctioning of the driver circuitry. Such malfunctioning is likely to shorten the lifetime of the LEDs due to a cascading malfunctioning effect.
The polarized capacitors 12 further introduce phase differences in the circuit which decreases power factor and the overall useful electrical power efficiency.
The opto-couplers 14 are typically expensive electrical components which will increase the overall cost of implementing the LED controller.
In addition to the above disadvantages, the Applicant submits that generic PWM controllers (whether analogue or digital) work based on fixed/narrow frequencies range and does not ensure that the controller is working in a discontinuous mode when the operating frequency changes.
It is thus an object of the invention to overcome, or at least ameliorate in part, one or more of the aforementioned problems.